The rheological response under simple shear of an active suspension of Escherichia coli is determined in a large range of shear rates and concentrations. The effective viscosity and the time scales characterizing the bacterial organization under shear are obtained. In the dilute regime, we bring evidences for a low shear Newtonian plateau characterized by a shear viscosity decreasing with concentration. In the semi-dilute regime, for particularly active bacteria, the suspension display a "super-fluid" like transition where the viscous resistance to shear vanishes, thus showing that macroscopically, the activity of pusher swimmers organized by shear, is able to fully overcome the dissipative effects due to viscous loss.Owing to its relevance in medicine, ecology and its importance for technological applications, the hydrodynamics of active suspensions is at the centre of many recent fundamental studies [1, 2]. In nature, wide classes of living micro-organisms move autonomously in fluids at very low Reynold numbers [3]. Their motility stems from a variety of propulsive flagellar systems powered by nanomotors. For bacteria such a bacillus subtilis or E.coli, the propulsion comes from the rotation of helix shaped flagella creating a propulsive force at the rear of the cell body [4]. Consequently, many original fluid properties stem from the swimming activity [5][6][7][8][9][10][11]. Due to hydrodynamic interactions bacteria may produce mesoscopic patterns of collective motion sometimes called "bio-turbulence" [13][14][15][16][17][18]. In a flow, these bacteria may organize spatially [12] and under shear, for pusher-swimmers, the swimming activity yields the possibility to decrease the macroscopic viscosity to values below the suspending fluid viscosity [5]. In the dilute regime, kinetic theories via a simple account of the dominant long range hydrodynamic field [19][20][21] provide closed-forms for shear viscosity as a function of shear rate. Remarkably, at low shear rate, these theories predict a Newtonian plateau with a viscosity decreasing linearly with concentration [19][20][21]. On the other hand, phenomenological theories were also proposed to describe macroscopically active suspensions via a coupling of hydrodynamic equations with polar and/or nematic order parameters [2, 5, 6,[22][23][24]. A striking outcome of these theories is that for a set of coupling parameters rendering essentially a high swimming activity, a self-organized motive macroscopic flow may show up in response to shear [22][23][24]. This onset of a dissipationless current is described in analogy with the super-fluidity transition [22,23] of liquids. Experimental evidences for viscosity reduction to values below the suspending fluid viscosity were brought for Bacillus subtilis [8] and E. coli [25] suspensions. However, no full rheological characterization (i.e. viscosity versus shear rate) under steady and uniform shear exists. Moreover, these pioneering experiments did not provide evidence for the low shear viscous plateau which is at the c...

The viscosity of an active suspension of E-Coli bacteria is determined experimentally in the dilute and semi dilute regime using a Y shaped micro-fluidic channel. From the position of the interface between the pure suspending fluid and the suspension, we identify rheo-thickening and rheo-thinning regimes as well as situations at low shear rate where the viscosity of the bacteria suspension can be lower than the viscosity of the suspending fluid. In addition, bacteria concentration and velocity profiles in the bulk are directly measured in the micro-channel.PACS numbers: 47.57.Qk,The fluid mechanics of microscopic swimmers in suspension have been widely studied in recent years. Bacteria [1, 2], algae [3,4] or artificial swimmers [5] dispersed in a fluid display properties that differ strongly from those of passive suspensions [6]. The physical relationships governing momentum and energy transfer as well as constitutive equations vary drastically for these suspensions [7,8]. Unique physical phenomena caused by the activity of swimmers were recently identified such as enhanced Brownian diffusivity [1,[8][9][10]] uncommon viscosity [4,12,13], active transport and mixing [11] or the extraction of work from isothermal fluctuations [13,16]. The presence of living and cooperative species may also induce collective motion and organization at the mesoscopic or macroscopic level [17,18] impacting the constitutive relationships in the semi-diluted or dense regimes. The E.Coli bacterium possesses a quite sophisticated propulsion apparatus consisting of a collection of flagella (7-10 µm length) organized in a bundle and rotating counter-clockwise [20]. In a fluid at rest, the wild-type strain used here has the ability to change direction by unwinding some flagella and moving them in order to alter its swimming direction (a tumble) approximately once every second [21]. In spite of the inherent complexity of the propulsion features, low Reynolds number hydrodynamics impose a long range flow field which can be modeled as an effective force dipole. Due to the thrust coming from the rear, E.coli are described as "pushers", hence defining a sign for the force dipole which has a crucial importance on the rheology of active suspensions [7]. For a dilute suspension of force dipoles, Haines et al [22] and Saintillan [24] derived an explicit relation relating viscosity and shear rate. They obtained an effective viscosity similar in form to the classical Einstein relation for dilute suspensions : η = η 0 (1 + Kφ) (η 0 being the suspending fluid viscosity and φ the volume fraction). These theories predict a negative value for the coefficient K for pushers at low shear rates, meaning the suspension can exhibit a lower viscosity than the suspending fluid. The theoretical assessment of shear viscosity relies on an assumed statistical representation of the orientations of the bacteria, captured by a Fokker-Plank equation and a kinematic model for the swimming trajectories [25,26].Despite the large number of theoretical studies, few experimen...

We investigate experimentally the emergence of collective motion in the bulk of an active suspension of Escherichia coli bacteria. When increasing the concentration from a dilute to a semi-dilute regime, we observe a continuous crossover from a dynamical cluster regime to a regime of 'bio-turbulence' convection patterns. We measure a length scale characterizing the collective motion as a function of the bacteria concentration. For bacteria fully supplied with oxygen, the increase of the correlation length is almost linear with concentration and at the largest concentrations tested, the correlation length could be as large as 24 bacterial body sizes (or 7-8 when including the flagella bundle). In contrast, under conditions of oxygen shortage the correlation length saturates at a value of around 7 body lengths.

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New processing techniques for manipulating blood and its components at a microfluidic scale are currently implemented. As for extracorporeal circulation, the in-line evaluation and monitoring of blood properties during these microfluidic techniques is a challenging task. Here, we show that the blood hematocrit can be measured non-invasively in a sub-millimeter medical tube using the non-Newtonian behavior of blood velocity profile. This hematocrit measurement is demonstrated on human blood with a simple Doppler ultrasound system. Results show a mean measurement error of 4.6 ± 1.3%Hct for hematocrit up to 52% and for 5 s-long ultrasonic signals. The simplicity and the measurement scale of the approach make it highly valuable for measuring hematocrit in new blood separation techniques. The approach may have an impact on in-vitro blood processing in general.

Purity, limited platelet activation, and preservation of platelet function are important stakes of preparation of platelet concentrates (PC) for clinical use. In fact, contaminating red blood cells and leukocytes, as well as activated and/or poorly functional platelets in PC, represents a risk of poor efficiency and adverse side effects during platelet transfusion. Therefore, optimization of preparation and storage of PC is still an active field of research. Shear-induced platelet activation is an unwanted side effect of the hard-spin (up to 5000g) step of centrifugation-based methods currently used in blood banks to prepare PC from whole blood samples. Here, we evaluated the effectiveness of an acoustic-based fractionation device for the isolation of human platelets from whole blood bags. The purity, activation status, and functionality of platelets isolated by acoustopheresis were compared with those of platelets isolated using a reference protocol known to produce limited platelet activation and consisting of two consecutive soft-spin centrifugations (120g and 1200g). Platelet concentration and purity were determined using an automated hematology analyzer. Platelet activation status and platelet reactivity to collagen and thrombin were assessed in flow cytometry by measurement of surface expression of P-selectin and activated integrin αIIbβ3. The ability of isolated platelets to incorporate into a thrombus when transfused to NOD/SCID mice was investigated by intravital microscopy using the ferric chloride-induced thrombosis model. Blood fractionation by acoustophoresis led to the elimination of more than 80% of red blood cells and leukocytes from the platelet fraction, whose mean purity was of 92.8 ± 12.8%. The activation status and reactivity to collagen and thrombin of acoustophoresis-isolated platelets were similar to those of platelets isolated by soft-spin centrifugation. Finally, acoustophoresis-isolated platelets were tethered, adhered to the vessel wall, and incorporated into a growing thrombus following ferric chloride-induced vascular injury. Together, our results indicate that acoustophoresis is a suitable method for the isolation of human platelets with minimal platelet activation and preservation of platelet function.

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